Significant attention has been placed on developing alternative energy sources to fossil fuels. One fossil fuel alternative having significant potential is biomass, particularly cellulosic biomass such as, for example, plant biomass. As used herein, the term “biomass” will refer to a living or recently living biological material. Complex organic molecules within biomass can be extracted and broken down into simpler organic molecules, which can subsequently be processed through known chemical transformations into industrial chemicals or fuel blends (i.e., a biofuel). In spite of biomass's potential in this regard, particularly plant biomass, an energy- and cost-efficient process that enables the conversion of biomass into such materials has yet to be realized.
Cellulosic biomass is the world's most abundant source of carbohydrates due to the lignocellulosic materials located within the cell walls of higher plants. Plant cell walls are divided into two sections: primary cell walls and secondary cell walls. The primary cell wall provides structural support for expanding cells and contains three major polysaccharides (cellulose, pectin, and hemicellulose) and one group of glycoproteins. The secondary cell wall, which is produced after the cell has finished growing, also contains polysaccharides and is strengthened through polymeric lignin covalently crosslinked to hemicellulose. Hemicellulose and pectin are typically found in abundance, but cellulose is the predominant polysaccharide and the most abundant source of carbohydrates. Collectively, these materials will be referred to herein as “cellulosic biomass.”
Plants can store carbohydrates in forms such as, for example, sugars, starches, celluloses, lignocelluloses, and/or hemicelluloses. Any of these materials can represent a feedstock for conversion into industrial chemicals or fuel blends. Carbohydrates can include monosaccharides and/or polysaccharides. As used herein, the term “monosaccharide” refers to hydroxy aldehydes or hydroxy ketones that cannot be further hydrolyzed to simpler carbohydrates. Examples of monosaccharides can include, for example, dextrose, glucose, fructose, and galactose. As used herein, the term “polysaccharide” refers to saccharides comprising two or more monosaccharides linked together by a glycosidic bond. Examples of polysaccharides can include, for example, sucrose, maltose, cellobiose, and lactose. Carbohydrates are produced during photosynthesis, a process in which carbon dioxide is converted into organic compounds as a way to store energy. This energy can be released when the carbohydrates are oxidized to generate carbon dioxide and water.
Despite their promise, the development and implementation of bio-based fuel technology has been slow. A number of reasons exist for this slow development. Ideally, a biofuel would be compatible with existing engine technology and have capability of being distributed through existing transportation infrastructure. Current industrial processes for biofuel formation are limited to fermentation of sugars and starches to ethanol, which competes with these materials as a food source. In addition, ethanol has a low energy density when used as a fuel. Although some compounds that have potential to serve as fuels can be produced from biomass resources (e.g., ethanol, methanol, biodiesel, Fischer-Tropsch diesel, and gaseous fuels, such as hydrogen and methane), these fuels generally require new distribution infrastructure and/or engine technologies to accommodate their physical characteristics. As noted above, there has yet to be developed an industrially scalable process that can convert biomass into fuel blends in a cost- and energy-efficient manner that are similar to fossil fuels.